A vehicle can be provided with an ultrasound sensor for detecting an obstacle around the vehicle. For example, as disclosed in JP-2004-264264A, a bumper of the vehicle is provided with a hole, through which the head portion of the ultrasound sensor is exposed to the exterior in such a manner that the outer surface of the head portion is arranged at the same surface with the outer surface of the bumper. In this case, the appearance of the vehicle is impaired because the head portion of the ultrasound sensor is exposed to the exterior of the vehicle.
In order to solve this problem, as disclosed in JP-10-123236A, a concave portion is arranged at the back side (inner side) of the bumper of the vehicle. A sensor of an ultrasound vibration type is housed in the concave portion to be invisible from the exterior of the vehicle. Thus, the appearance of the vehicle is maintained.
In this case, the ultrasound sensor (ultrasound-vibration-type sensor) has an ultrasound vibrator, which is made of a ceramic and fixed to a metal base. The ultrasound sensor is directly mounted to the concave portion formed at the back surface of the vehicle bumper.
Therefore, the ultrasound sensor is exposed to the environmental air, although the ultrasound sensor is arranged at the vehicle inner side of the vehicle bumper. Especially, there will occur corrosion at the metal base and wiring cords and the like of the ultrasound sensor. Accordingly, it is desirable to accommodate the ultrasound senor in a casing (housing) so as to protect the ultrasound sensor from the environmental air, and to attach the ultrasound sensor to the back surface of the vehicle bumper.
However, in the case where the ultrasound sensor is accommodated in the casing, the ultrasound will be transferred through the casing and the vehicle bumper when the ultrasound vibrator sends/receives the ultrasound. Thus, the ultrasound is transferred to a relatively broad region of the vehicle bumper, through the case.
In the case where the ultrasound is transferred to the relatively broad region of the vehicle bumper, the vibration at the different positions (e.g., middle portion and periphery portion of the region) will be canceled out and/or interfer due to different phases of vibration occurring at the different positions. Therefore, when ultrasound generated by the ultrasound vibrator is transferred to the vehicle bumper through the casing, directivity of the ultrasound sensor is narrowly limited and becomes irregular.